In future radio systems implementing flexible spectrum use, with self organizing/self optimizing features for optimized local area access, it is expected that spectrum will be shared with other radio systems. Deployment in unlicensed bands, for example the ISM bands, is very likely.
For a radio system that is scalable to achieve top performance in widely varying node densities, a node must be able to sense, whether a radio resource is in use by another node of a cooperating radio system. The effects of interference ultimately show at a receiving node, which may be overcome by having the receiver transmit beacon signals indicating its presence. For example, a wireless local area network (WLAN) optionally uses request to send (RTS) and clear to send (CTS) messages as broadcast beacons that enable both the sender and the receiver to claim the use of the wireless medium. Another approach is the transmission of a “busy tone”, or “busy burst” by a node.
In a link with a balanced volume of data in both directions, the uplink transmission by an intended receiving node can serve to mark the radio resource as used for reception of the downlink data, and vice versa.
For example, consider a mobile phone streaming video or high-quality audio. It receives large volumes of downlink traffic on a high number of radio resources (i.e. bandwidth, time slots), while transmitting only negligible amounts of uplink control traffic that may fit into the smallest radio resource unit that can be allocated. Still, it needs to announce its use of the downlink radio resources to other, nearby radios, to prevent them using the same resource for transmission and causing interference. This can be done by broadcasting regular special signals on the radio resources used for reception. The transmission of the special signal like for example beacon signals on a radio resource channel by a receiving node marks it occupied to other, nearby radios.